Effects of hydrothermal pretreatments on the anaerobic digestion of pig manure and ecological safety of biogas slurry

Abstract: Biogas production via anaerobic digestion (AD) can effectively utilize pig manure to produce methane instead of fossil energy. However, a large amount of lignocellulose contained in the pig manure cannot be fully decomposed during hydrolysis in the methane production during AD. Therefore, it is necessary to adopt the appropriate pretreatment for the better hydrolysis and AD performance of pig manure. It is also a high demand to consider the ecological safety index (e.g. heavy metals Cr, As, Cd, Hg, and Pb, Escherichia coli) of anaerobic digestates as agricultural organic fertilizers. A hydrothermal treatment can be widely expected to improve the biodegradability of materials, where the hemicellulose can be deconstructed to dissolve the organic matter at high temperature and pressure. Thus, it is supposed that the hydrothermal treatment of pig manure can be extended for the subsequent methane conversion. In this study, a systematic investigation was conducted to clarify the effects of hydrothermal pretreatment at different temperatures (70, 90, 120, 150, and 170 ℃) on the physicochemical properties and AD performance of pig manure, together with the ecological safety of AD digestate from pig manure. The results showed as follows. The total solid (TS) in the pig manure decreased by 1.5%-8.7%, whereas, the Soluble Chemical Oxygen Demand (SCOD) increased by 3.9%-43.6%, with the increasing hydrothermal temperature. It infers that the hydrothermal treatment enhanced the biodegradability and hydrolysis of pig manure. Nevertheless, it was easy to produce hardly degraded substances, such as organic humic acid and fulvic acid, during the hydrothermal process of pig manure. The organic humic acids and fulvic acids in the pig manure gradually increased with the increasing hydrothermal temperature. The relative content of fulvic acids increased from 37.63% to 41.97%, when the temperature increased from 150 to 170 ℃. The cumulative biogas and methane yields were 602-677 mL/g and 350-398 ml/g, respectively, in the pig manure that was pretreated at different temperatures after 30-day AD. The highest methane yield of 398±40 mL/g was obtained from the pig manure after 150 ℃ pretreatment, which significantly increased by 5.6% (P < 0.05), compared with the untreated. There was no significant enhancement in the methane production for the hydrothermal pretreatment at the rest temperatures. The reason was that the low pretreatment temperature (70 and 90 ℃) slightly increased the SCOD and hydrolysis. Maillard reaction inhibited the subsequent AD process for the less biodegrade substances at the higher temperature of 170 ℃. The contents of heavy metals Cr, As, Cd, Hg, and Pb in the pig manure digestate fully met the requirements in the national standard of Agricultural Biogas Slurry (GB/T 40750-2021). The content of Escherichia coli met the Hygienic Requirements for Harmless Disposal of Night Soil (GB 7959-2012). The content of oxytetracycline in the pig manure digestate was reduced effectively, while the enrofloxacin, sulfadiazine, and norfloxacin were lower than the detection limit. Therefore, appropriate hydrothermal pretreatment (150 ℃) of pig manure can improve the methane production performance during AD, further reducing the harmless treatment of manure and the environmental risk of AD digestate that is applied to the field.